CN110809632B - Method for spot welding of martensitic stainless steel plate - Google Patents
Method for spot welding of martensitic stainless steel plate Download PDFInfo
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K11/00—Resistance welding; Severing by resistance heating
- B23K11/10—Spot welding; Stitch welding
- B23K11/11—Spot welding
- B23K11/115—Spot welding by means of two electrodes placed opposite one another on both sides of the welded parts
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K11/00—Resistance welding; Severing by resistance heating
- B23K11/16—Resistance welding; Severing by resistance heating taking account of the properties of the material to be welded
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K2101/00—Articles made by soldering, welding or cutting
- B23K2101/006—Vehicles
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K2101/00—Articles made by soldering, welding or cutting
- B23K2101/18—Sheet panels
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K2103/00—Materials to be soldered, welded or cut
- B23K2103/02—Iron or ferrous alloys
- B23K2103/04—Steel or steel alloys
- B23K2103/05—Stainless steel
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
Abstract
Method for welding two steel plates, the steel plates having a thickness of 0.10 to 6.0mm and a composition as follows: c is more than or equal to 0.005% and less than or equal to 0.3%; mn is more than or equal to 0.2 percent and less than or equal to 2.0 percent; trace amount of Si is less than or equal to 1.0 percent; trace amount of S is less than or equal to 0.01 percent; trace amount of P is less than or equal to 0.04 percent; cr is more than or equal to 10.5 percent and less than or equal to 17.0 percent; trace amount of Ni is less than or equal to 4.0 percent; trace amount of Mo is less than or equal to 2.0 percent; mo +2 xW is less than or equal to 2.0 percent; trace amount of Cu is less than or equal to 3 percent; trace amount of Ti is less than or equal to 0.5 percent; trace amount of Al is less than or equal to 0.2 percent; o is less than or equal to 0.04 percent in trace amount; nb is more than or equal to 0.05 percent and less than or equal to 1.0 percent; nb + Ta is more than or equal to 0.05% and less than or equal to 1.0%; the ratio of (Nb + Ta)/(C + N) is more than or equal to 0.25% and less than or equal to 8; v is less than or equal to 0.3 percent when the trace amount is less than or equal to V; trace amount is less than or equal to Co and less than or equal to 0.5 percent; trace amount is less than or equal to Cu, Ni and Co is less than or equal to 5.0 percent; trace amount of Sn is less than or equal to 0.05 percent; trace amount is less than or equal to B and less than or equal to 0.1 percent; trace Zr is less than or equal to 0.5 percent; ti + V + Zr is less than or equal to 0.5 percent; trace amount of H is less than or equal to 5 ppm; trace amount is less than or equal to N and less than or equal to 0.2 percent; (Mn + Ni) ≥ Cr-10.3-80 x [ (C + N)2]) (ii) a Trace amount of Ca is less than or equal to 0.002%; trace amount is less than or equal to rare earth element and/or Y is less than or equal to 0.06 percent; the balance of iron and impurities generated in the production process; the temperature (Ms) of the steel plate is more than or equal to 200 ℃; the temperature (Mf) of the steel plate is more than or equal to-50 ℃; the microstructure of the steel sheet comprises not more than 0.5% of carbides and not more than 20% of residual ferrite, the remainder being martensite; it is characterized in that In that it comprises the following steps: a first welding step, of duration (t), in ms, and with a clamping force (F), in daN: for a thickness (e) of 0.10 to 0.50 mm: t ═ 40 × e +36 ± 10%; for a thickness (e) of 0.51 to 1.50 mm: t ═ (124 × e-13) ± 10%; for a thickness (e) of 1.51 to 6.0 mm: t ═ 12 × e +47 ± 10%; for a thickness (e) of 0.10 to 1.50mm, F ═ 250 × e +90 ± 10%; for a thickness (e) of 1.51mm to 6.0 mm: f ═ 180 × e +150) ± 10%, where (e) is the thickness of each of the steel plates or the thickness of the thinnest steel plate thereof; in this step, a current is applied between the welding electrodes, the intensity of which is between 80% and 100% of the maximum allowable intensity corresponding to the molten metal tapping; a second step, the strength is 0 to 1 kA; and a third step, with an intensity of between 3.5kA and 4.5kA, for a time of at least 755 ms.
Description
Technical Field
The present invention relates to the steel industry and more particularly to a method for spot welding of steel sheets.
Background
Hot-worked martensitic stainless steel sheets are known with adjustments to the composition, initial microstructure and heat treatment parameters, which allow to obtain high mechanical properties and a strong ability to complex shapes. Such steel sheets are described in the applicant's document PCT/IB2017/051636 and are mainly used in the automotive industry.
The compositions in weight percentage are as follows:
0.005％≤C≤0.3％；
0.2％≤Mn≤2.0％；
trace amount of Si is less than or equal to 1.0 percent;
trace amount of S is less than or equal to 0.01 percent;
trace amount of P is less than or equal to 0.04 percent;
cr is more than or equal to 10.5 percent and less than or equal to 17.0 percent; preferably, 10.5 percent to 14.0 percent of Cr;
trace amount of Ni is less than or equal to 4.0 percent;
trace amount of Mo is less than or equal to 2.0 percent;
Mo+2×W≤2.0％；
trace amount of Cu is less than or equal to 3 percent; preferably, the trace amount is less than or equal to Cu and less than or equal to 0.5 percent;
trace amount of Ti is less than or equal to 0.5 percent;
trace amount of Al is less than or equal to 0.2 percent;
o is less than or equal to 0.04 percent in trace amount;
0.05％≤Nb≤1.0％
0.05％≤Nb+Ta≤1.0％；
0.25％≤(Nb+Ta)/(C+N)≤8；
v is less than or equal to 0.3 percent when the trace amount is less than or equal to V;
trace amount is less than or equal to Co and less than or equal to 0.5 percent;
trace amount is less than or equal to Cu, Ni and Co is less than or equal to 5.0 percent;
trace amount of Sn is less than or equal to 0.05 percent;
trace amount is less than or equal to B and less than or equal to 0.1 percent;
trace Zr is less than or equal to 0.5 percent;
Ti+V+Zr≤0.5％；
trace ≦ H ≦ 5ppm, preferably, trace ≦ H ≦ 1 ppm;
trace amount is less than or equal to N and less than or equal to 0.2 percent;
(Mn+Ni)≥(Cr-10.3–80×[(C+N)2])；
trace amount of Ca is less than or equal to 0.002%;
trace amount is less than or equal to rare earth element and/or Y is less than or equal to 0.06 percent;
the balance of iron and processing impurities;
the initial temperature (Ms) of the martensite transformation of the steel plate is more than or equal to 200 ℃;
and the termination temperature (Mf) of the martensitic transformation of the steel plate is more than or equal to-50 ℃.
The microstructure of the initial steel sheet obtained after suitable means (possibly including hot working and/or cold working) consists of ferrite and/or tempered martensite and 0.5 to 5% by volume of carbides, and the ferrite grain size is 1 to 80 μm, preferably 5 to 40 μm. The thickness of the initial steel plate is 0.1mm to 10mm, more typically 0.1mm to 6 mm.
The treatment process usually applied thereto starts with austenitizing of the steel sheet, i.e. raising the temperature above the Ac1 temperature of the steel to form austenite, replacing ferrite and carbides constituting the initial microstructure, and is carried out under conditions that limit decarburization and surface oxidation of the steel sheet as much as possible. Typically, no more than 20% residual ferrite and no more than 0.5% carbides are present.
The steel sheet is then subjected to several (at least two) successive hot forming steps under time and temperature conditions so as to maintain the structure of low ferrite and carbide content obtained after austenitization throughout the forming process. These hot forming operations are carried out at a temperature above the martensitic transformation start temperature Ms. If desired, the temperature may be reheated or maintained by heating the tool between or during each hot forming operation so that the temperature of the steel sheet does not fall below Ms between the forming and forming operations (during transfer of the steel sheet from one tool to another or during a change in configuration of the tool if the steel sheet remains on the same tool).
It should be understood that the term "thermoforming step" is meant to include deformation or material removal operations such as deep drawing, hot drawing, stamping, cutting, punching, which steps may be performed in any order chosen by the manufacturer.
After the hot forming, the resultant member is cooled, and the cooling conditions are not particularly limited.
On cooling, a cutting step or a final hot forming step may be carried out between Ms and Mf (the termination temperature of the martensitic transformation), provided that the microstructure consists of at least 10% austenite and not more than 20% ferrite, the remainder being martensite.
The steel sheet thus obtained has strong mechanical properties at ambient temperature, in particular due to the high martensite content. Typically, the tensile strength Rm is at least 1000MPa, the yield strength Re is at least 800MPa, the elongation after break A is at least 8% measured according to standard ISO 6892, the thickness is 1.5mm and the bending angle is at least 60 ° measured according to standard VDA 238-. This means that the steel sheet finally obtained has excellent formability and is particularly useful in the automotive industry or in the aeronautical, architectural or railway industry for forming parts with structural functions.
Finally, after cooling to ambient temperature after the last forming operation, the microstructure of the steel sheet comprises not more than 0.5% volume fraction of carbides and not more than 20% volume fraction of residual ferrite, the remainder being martensite.
Such steel sheets have a typical thickness of 0.10mm to 6.0mm, and have a disadvantage in that weldability is sometimes considered to be insufficient when welding is performed using a spot welding method under the conditions most commonly used by vehicle manufacturers. It has been found that in the weld zone it is not easy to obtain a crosshead tensile strength sufficient for a given thickness of steel sheet (i.e. typically at least 450daN for a thickness of 1.2mm of steel sheet): the material is too weak at the weld.
It is possible to improve the results by varying the welding parameters, i.e. by adding a post-heating pulse to a standard welding cycle (usually for martensitic steels), but the optimization carried out so far has not resulted in a welding cycle having a duration of less than 5s to obtain satisfactory welding quality. This time is too long for vehicle manufacturers who must be able to weld these sheets, and they must be aware of the productivity limitations they face when using them for vehicle mass production. Their goal is a total time of the welding cycle of no more than about 1 s. A welding cycle with a total time of 1.5s, or even 2s, is sometimes acceptable.
Disclosure of Invention
The aim of the present invention is therefore to propose a spot welding cycle which is particularly suitable for the use of the aforementioned martensitic stainless steel sheets for hot drawing and which allows such welding to be carried out under conditions industrially suitable for the automotive industry.
To this end, the subject of the invention is a method for welding two stainless steel plates having a thickness of 0.10mm to 6.0mm and having the following composition in weight percent:
0.005％≤C≤0.3％；
0.2％≤Mn≤2.0％；
trace amount of Si is less than or equal to 1.0 percent;
trace amount of S is less than or equal to 0.01 percent;
Trace amount of P is less than or equal to 0.04 percent;
cr is more than or equal to 10.5 percent and less than or equal to 17.0 percent; preferably, 10.5 percent to 14.0 percent of Cr;
trace amount of Ni is less than or equal to 4.0 percent;
trace amount of Mo is less than or equal to 2.0 percent;
Mo+2×W≤2.0％；
trace amount of Cu is less than or equal to 3 percent; preferably, the trace amount is less than or equal to Cu and less than or equal to 0.5 percent;
trace amount of Ti is less than or equal to 0.5 percent;
trace amount of Al is less than or equal to 0.2 percent;
o is less than or equal to 0.04 percent in trace amount;
0.05％≤Nb≤1.0％；
0.05％≤Nb+Ta≤1.0％；
0.25％≤(Nb+Ta)/(C+N)≤8；
v is less than or equal to 0.3 percent when the trace amount is less than or equal to V;
trace amount is less than or equal to Co and less than or equal to 0.5 percent;
trace amount is less than or equal to Cu, Ni and Co is less than or equal to 5.0 percent;
trace amount of Sn is less than or equal to 0.05 percent;
trace amount is less than or equal to B and less than or equal to 0.1 percent;
trace Zr is less than or equal to 0.5 percent;
Ti+V+Zr≤0.5％；
trace ≦ H ≦ 5ppm, preferably, trace ≦ H ≦ 1 ppm;
trace amount is less than or equal to N and less than or equal to 0.2 percent;
(Mn+Ni)≥(Cr-10.3–80×[(C+N)2])；
trace amount of Ca is less than or equal to 0.002%;
trace amount is less than or equal to rare earth element and/or Y is less than or equal to 0.06 percent;
the balance of iron and processing impurities;
the starting temperature (Ms) of the martensitic transformation of the steel plate is more than or equal to 200 ℃;
the termination temperature (Mf) of the martensitic transformation of the steel plate is more than or equal to-50 ℃;
the microstructure of the steel sheet comprises not more than 0.5% volume fraction of carbides and not more than 20% volume fraction of residual ferrite, the remainder being martensite;
the method is characterized by comprising the following steps:
a first welding step, duration (t), in ms, and with a clamping force (F), in daN,
for a thickness (e) of 0.10mm to 0.50 mm: t ═ 40 × e +36) ± 10%,
For a thickness (e) of 0.51mm to 1.50 mm: t ═ 124 × e-13) ± 10%,
for a thickness (e) of 1.51mm to 6.0 mm: t ═ 12 × e +47 ± 10%,
for a thickness (e) of 0.10mm to 1.50 mm: f ═ 250 × e +90) ± 10%,
for a thickness (e) of 1.51mm to 6.0 mm: f ═ 180 × e +150) ± 10%,
wherein (e) is the thickness of each of the steel plates or the thickness of the thinnest steel plate among them,
in this step, a current having an intensity of 80% to 100% of the maximum allowable intensity corresponding to the molten metal discharge is applied between the welding electrodes;
a second step in which the current intensity is set to 0 to 1 kA; and
a third step in which the passage of current is resumed at an intensity of 3.5kA to 4.5kA for a time of at least 755ms to heat-treat the welded region.
Preferably, in the second step, the flow of current in the welding area is interrupted.
Advantageously, the sum of the times of said first, second and third steps does not exceed 2s, preferably 1.5s, most preferably 1 s.
The steel sheet may be a hot rolled steel sheet.
It will be understood that in the present invention, the steel sheet, preferably considered and having the above composition, is subjected to a spot welding cycle, which is in particular a function of the choice of the parameters and of the sequence of the operations.
It is to be noted that the conditions for performing spot welding are sufficiently defined by:
the pressure exerted by the welding electrode on the parts to be welded, which pressure, together with the chemical composition and the surface roughness of the parts, affects the contact resistance;
the current intensity flowing through the area to be welded, which is supplied by a regulated power supply, according to other parameters that cannot be strictly controlled;
welding time or time of different steps thereof.
Therefore, the potential difference between the two steel plates varies according to the contact resistance, and thus according to the energy injected into the welding region. The potential difference and the power themselves do not directly represent the parameters of the method, but they become the parameters to be applied due to the controllable and controlled operating conditions represented by the clamping force and the current intensity.
Welding starts with a first step in which an electric current of regulated intensity is passed through the steel plates to be welded, which are brought into contact with each other in advance under the action of a force. The force to be applied and the current transit time are generally dictated by the standard (e.g., SEP1220 or ISO18278-2) that the user wishes to employ. Once these two parameters are selected and used, the user changes the welding current until molten metal is discharged, which represents the maximum intensity value within the weldability range. The welding current intensity of the present invention is in the range of 80% to 100% of the maximum intensity. Generally, in the present invention, when the steel sheet to be welded has a thickness of 1.2mm, the welding current intensity is 5.5 kA. Generally, the maximum allowable weld strength corresponding to molten metal expulsion is obtained experimentally by standardized methods, see for example the standards SEP1220 and ISO 18278-2. Thus, in each particular case of the practice of the invention, the skilled person must determine the particular welding process of the invention when he finally determines it. However, this determination is not a typical feature of the present invention and similarly, problems of optimizing the welding amperage may be encountered when implementing any spot welding method and is generally performed as described.
For thicknesses of 0.1mm to 1.50mm, the force F (in daN) is represented by the following equation:
F＝250×e+90，
for a thickness of 1.51mm to 6mm, the equation is: f is 180 × e +150,
e is the thickness of the two welded steel plates or, if they have different thicknesses, the thickness of the thinnest one of them.
Variations in force F are allowed within ± 10% of these representation values.
For thicknesses of 0.10mm to 0.50mm, the welding time t (in ms) can also be expressed by the following equation: t is 40 × e +36,
for a thickness of 0.51mm to 1.50 mm: t is 124 × e-13, and
for a thickness of 1.51mm to 6.0 mm: t is 12 × e +47,
variations within ± 10% of these representative values are permissible.
In a second step, the pressure of the electrodes is maintained, the passage of current is stopped or greatly reduced, and an intensity not exceeding 1kA, ideally 0kA, is applied within a minimum time tf (in ms) expressed by the following equation: tf is more than or equal to 34 × e + 2.
This causes the steel sheet in the weld zone to suddenly cool to a temperature between Ac1 and Ac5, which results in re-austenitization of the zone.
In a third step, the current is restored to a value of between 3.5kA and 4.5kA, in order to maintain the temperature between Ac1 and Ac5 and to heat treat the welded zone, which will change its structural characteristics, giving it the desired mechanical properties. The third step must last at least 755ms to ensure effectiveness and does not specify a maximum time. The longer the time, the more effective the heat treatment to ensure a higher crosshead tensile strength. However, it is advantageous to avoid excessively lengthening this third step, so as not to lengthen the welding cycle for a length of time incompatible with the requirements of industrial production. Advantageously, as mentioned above, preferably the total time of the three welding cycle steps does not exceed 2s, preferably 1.5s, and optimally 1 s.
If this treatment is carried out under the conditions described, it is possible to obtain crosshead tensile strengths of sufficient value for the thickness of the steel sheet considered, even exceeding these values, and welding cycle times of about 1s, even shorter, and therefore compatible with the current industrial requirements of the automotive industry for mass production of vehicles. Thus, under good economic conditions, spot-welded steel sheets can benefit from the advantages of the method described in PCT/IB2017/051636, which involves easily obtaining parts of complex shape with high mechanical properties and a well-defined composition of the hot-formed martensitic stainless steel.
Drawings
The invention will be better understood by reading the following description, given with reference to the following drawings:
FIG. 1 shows a photomicrograph of the weld zone when two steel plates are welded together using a method not in accordance with the invention;
FIG. 2 shows a detail of the weld area of FIG. 1;
fig. 3 gives a micrograph of the welded area after the second step of the method of the invention, thus in an intermediate state before the third welding step, and shows the disappearance of residual ferrite at this stage;
fig. 4 shows a micrograph of the welded area after the method according to the invention has been carried out completely.
Fig. 5 shows a detail of the welding area in fig. 4.
Detailed Description
The inventors carried out an experiment of welding two steel plates having the following composition in weight percent: cr is 11.02%; nb is 0.11%; mn is 0.50%; c ═ 0.059%; n is 0.0107%; the remainder being iron and processing impurities, in an austenitic state and pressure quenched, thus according to the invention described in PCT/IB2016/052302, and having a thickness of 1.2mm, the following results were obtained.
In a first series of experiments, a conventional welding cycle with a total duration of 560ms was used, in which a current of intensity 5.5kA was passed between the electrodes for 280ms at a pressure of 4000N, followed by a zero intensity phase of 280ms, during which the pressure was kept constant (parameters specified by standard ISO 18278-2 and generally used by vehicle manufacturers). The results are shown in fig. 1 and 2, and fig. 1 and 2 show micrographs of the welded area. In the center of fig. 1, the heat affected zone HAZ corresponding to the actual welded fusion zone 1 and its surroundings can be seen. The boundary of the melted zone 1 is a crack 2 growing in a large grain size HAZ 3, and white ferrite 4 (also clearly visible in fig. 1) is visible in the HAZ 3. It is this brittle ferrite 4 that causes the generation of cracks 2 and thus poor tensile strength of the crosshead. The proportion of ferrite in the HAZ 3 is 20% to 80% by area, which is significantly higher than would be expected from the reading of the equilibrium map. The crosshead tensile strength measured was 290daN and thus largely insufficient to meet the requirements of e.g. car manufacturers.
Shortening the current implementation time (from 280ms to 140ms) is beneficial because such shortening allows the range of large grain size HAZ 3 to be reduced and the percentage of residual ferrite to be reduced without significantly altering the melt zone 1. However, HAZ 3 still contains a large amount of brittle ferrite, and the crosshead tensile strength is not sufficiently improved.
In a second series of experiments according to the invention, the passage of current was interrupted for 46ms after the same first step as in the previous experiments, while maintaining the pressure of the electrodes. And in the previous experiment a third step was added, in which a recovery current was passed at an intensity of 4kA for 814ms to heat treat the welded area.
In summary, the loop in the embodiment of the present invention lasts for 140+46+814 for 1000 ms.
The aim is to obtain a weld of the two parts that does not exhibit a weak assembly point, in other words the cross-head tensile strength of the weld zone must be sufficient to meet this aim, and such a weld is obtained within a total cycle time that is capable of ensuring satisfactory plant productivity under industrial conditions. Generally, as in the described embodiment, a welding cycle time of about 1s is the satisfactory result for mass production of welded steel plates in the automotive industry.
Fig. 3 shows the appearance of a weld zone that can be obtained with the invention after the second step of the method of the invention lasting only 46 ms. Fig. 4 and 5 show the weld zone after the entire method of the invention has been carried out. Not only does the large grains in the HAZ 3 disappear in fig. 4, but also the toughness of the HAZ 3 and the molten zone 1 causes the cracks 2 to deflect into the matrix metal 5, the traces of the cracks 2 being visible in fig. 5.
In this way, a crosshead tensile strength of more than 450daN is obtained at the weld, which in the described embodiment is the target that has been set according to the thickness of the steel plates to be welded.
The inventors attribute the advantages of the method of the present invention over the more traditional spot welding methods to the sum of the following factors, which appear to have an unexpectedly significant synergistic effect.
Performing the first rapid welding cycle may reduce the residence time above the Ac5 point and minimize separation of gamma and alpha elements that result in large grain ferrite formation in the HAZ 3. It was thus found that the white ferrite 4 in fig. 1 had completely disappeared from the HAZ 3 in fig. 3.
In a second step, the current circulation is interrupted (or at least the current intensity is sharply reduced) and the weld zone is cooled to a re-austenitizing temperature of around 900 ℃.
The third step of recovering the current at a relatively high intensity, although lower than the first step, eventually eliminates the residual large-grain ferrite present in the HAZ around the footprint and provides satisfactory mechanical properties (fig. 4 and 5). It can also be seen that the crack 2 in fig. 4 is no longer along the HAZ as in fig. 1, but is deflected into the matrix metal 5 in fig. 4, leaving a large diameter spot on one of the two steel plates.
The steel sheet used in the practice of the present invention may be hot rolled or cold rolled. It is important firstly that their composition and microstructure must comply with what has been said before, and secondly that their thickness is within a range that allows spot welding, and therefore is generally between 0.10mm and 6.0 mm.
Claims (9)
1. A method for welding two steel plates having a thickness of 0.10 to 6.0mm and having the following composition in weight percent:
0.005％≤C≤0.3％；
0.2％≤Mn≤2.0％；
trace amount of Si is less than or equal to 1.0 percent;
trace amount of S is less than or equal to 0.01 percent;
trace amount of P is less than or equal to 0.04 percent;
10.5％≤Cr≤17.0％；
trace amount of Ni is less than or equal to 4.0 percent;
trace amount of Mo is less than or equal to 2.0 percent;
Mo+2×W≤2.0％；
trace amount of Cu is less than or equal to 3 percent;
trace amount of Ti is less than or equal to 0.5 percent;
trace amount of Al is less than or equal to 0.2 percent;
o is less than or equal to 0.04 percent in trace amount;
0.05％≤Nb≤1.0％；
0.05％≤Nb+Ta≤1.0％；
0.25％≤(Nb+Ta)/(C+N)≤8；
v is less than or equal to 0.3 percent when the trace amount is less than or equal to V;
trace amount is less than or equal to Co and less than or equal to 0.5 percent;
trace amount is less than or equal to Cu, Ni and Co is less than or equal to 5.0 percent;
Trace amount of Sn is less than or equal to 0.05 percent;
trace amount is less than or equal to B and less than or equal to 0.1 percent;
trace Zr is less than or equal to 0.5 percent;
Ti+V+Zr≤0.5％；
trace amount of H is less than or equal to 5 ppm;
trace amount is less than or equal to N and less than or equal to 0.2 percent;
(Mn+Ni)≥(Cr-10.3–80×[(C+N)2])；
trace amount of Ca is less than or equal to 0.002%;
trace amount is less than or equal to rare earth element and/or Y is less than or equal to 0.06 percent;
the balance of iron and processing impurities;
the starting temperature (Ms) of the martensitic transformation of the steel plate is more than or equal to 200 ℃;
the termination temperature (Mf) of the martensitic transformation of the steel plate is more than or equal to-50 ℃;
the microstructure of the steel plate contains carbide with volume fraction not more than 0.5%, residual ferrite with volume fraction not more than 20%, and martensite in balance;
characterized in that the method comprises the following steps:
a first welding step, duration t, in ms, and with a clamping force F, in daN,
for a thickness e of 0.10mm to 0.50 mm: t ═ 40 × e +36) ± 10%,
for a thickness e of 0.51mm to 1.50 mm: t ═ 124 × e-13) ± 10%,
for a thickness e of 1.51mm to 6.0 mm: t ═ 12 × e +47 ± 10%,
for a thickness e of 0.10mm to 1.50 mm: f ═ 250 × e +90) ± 10%,
for a thickness e of 1.51mm to 6.0 mm: f ═ 180 × e +150) ± 10%,
wherein e represents a thickness of each of the steel plates or a thickness of the thinnest steel plate among the steel plates,
in this step, a current having an intensity of 80% to 100% of the maximum allowable intensity corresponding to the molten metal discharge is applied between the welding electrodes;
A second step in which the current intensity is set to 0 to 1 kA; and
a third step in which the passage of current is resumed at an intensity of 3.5kA to 4.5kA for a time of at least 755ms to heat-treat the welded region.
2. The method of claim 1, wherein 10.5% Cr is less than 14.0%.
3. The method of claim 1, wherein the trace amount is ≦ Cu ≦ 0.5%.
4. The method of claim 1, wherein the trace amount H is less than or equal to 1 ppm.
5. Method according to claim 1, characterized in that in the second step the circulation of the current in the welding area is interrupted.
6. The method of claim 1, wherein the sum of the times of the first, second and third steps does not exceed 2 s.
7. The method of claim 1, wherein the sum of the times of the first, second and third steps does not exceed 1.5 s.
8. The method of claim 1, wherein the sum of the times of the first, second and third steps does not exceed 1 s.
9. The method of claim 1, wherein the steel sheet is a hot rolled steel sheet.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| PCT/IB2017/053975 WO2019002924A1 (en) | 2017-06-30 | 2017-06-30 | Method for welding using points of martensitic stainless steel sheets |
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| CN110809632A CN110809632A (en) | 2020-02-18 |
| CN110809632B true CN110809632B (en) | 2021-10-08 |
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| US (1) | US11478873B2 (en) |
| EP (1) | EP3645764B1 (en) |
| JP (1) | JP7053688B2 (en) |
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| CA3001971A1 (en) * | 2015-10-21 | 2017-04-27 | Nippon Steel & Sumitomo Metal Corporation | Method of resistance spot welding a plurality of steel sheets |
| CN112442634B (en) * | 2020-11-04 | 2022-04-22 | 中航卓越锻造（无锡）有限公司 | High-strength high-toughness large martensitic stainless steel ring forging and manufacturing method thereof |
| JP7355280B1 (en) | 2022-03-25 | 2023-10-03 | Ｊｆｅスチール株式会社 | Resistance spot welding joint and its manufacturing method |
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- 2017-06-30 CN CN201780092738.7A patent/CN110809632B/en active Active
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| Publication number | Publication date |
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| KR20200027476A (en) | 2020-03-12 |
| KR102279421B1 (en) | 2021-07-20 |
| CN110809632A (en) | 2020-02-18 |
| WO2019002924A1 (en) | 2019-01-03 |
| EP3645764A1 (en) | 2020-05-06 |
| US11478873B2 (en) | 2022-10-25 |
| US20200223008A1 (en) | 2020-07-16 |
| BR112019028089A2 (en) | 2020-07-28 |
| BR112019028089B1 (en) | 2023-01-10 |
| JP7053688B2 (en) | 2022-04-12 |
| EP3645764B1 (en) | 2022-01-19 |
| JP2020529921A (en) | 2020-10-15 |
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